The bone tool is a tool which allows you to make fluid animations (like joints). To start using the bone tool, first you need to draw a limb (leg,arm, torso). Select the limb with the select tool, then go to the bone tool. Click with the bone tool on one end of the limb, click again where you want to make a joint, and then one more click where you want to end the limb. Go to your timeline, and make a keyframe further down the line. Right click on the keyframe, and insert pose. Go back to your limb, and move it anyway you want!
http://www.youtube.com/watch?v=6wylebW68c4